The invention relates to a method and apparatus for controlling a fan motor according to the preambles of the independent claims.
In the cooling of devices with a high dissipation power, it is common to use fans which generate air movement which is used to transfer heat away from the object producing it. For example, centrifugal fans provided with an induction motor implemented by an external rotor are employed in cooling due to their efficiency, simplicity and affordability. The rotation speed of such machines provided with external rotors can typically adjusted to a speed lower than the nominal speed by changing the effective value of supply voltage either by a transformer or a thyristor clipper. The nominal rotation speed cannot, however, be increased since the supply network frequency determines the synchronous speed of the induction motor. Furthermore, commercially available variable transformers and thyristor clippers are relatively expensive components in proportion to the fan price. In addition, these speed control devices can be operated only manually, which limits their use and practicability.
External rotor fans are often connected to a supply network without any control device, in which case their intake power is constant regardless of the real cooling demand. Cooling fans are typically used 24 hours in a day, and thus the energy consumption is maximal with respect to the cooling demand.
U.S. Pat. No. 5,963,887 discloses a cooling solution where the rotation speed of a fan is controlled according to the temperature of the object to be cooled by determining certain operating points for the rotation speed of the fan. This kind of use restricts the controllability of the fan in an undesired manner.